Agriculture Reference
In-Depth Information
Urea is very soluble and may leach with irrigation water to some depth be-
fore it undergoes hydrolysis in water. The hydrolysis reactions that occur are
(NH
2
)
2
CO
2H
2
O
(NH
4
)
2
CO
3
(5.4)
(NH
4
)
2
CO
3
2H
2
O
2NH
4
OH
H
2
CO
3
(5.5)
The NH
4
OH produced by reaction 5.5 readily dissociates into NH
4
and OH
ions, and the former ions are subsequently oxidized to NO
3
. Thus, acidification
through nitrification can occur at depths of 40-60 cm in soil under drippers, es-
pecially if a large “slug” of urea is applied. Acidity at depth is difficult to correct
by liming (section 5.5.3).
Gaseous Losses of N
The NH
4
OH formed by the hydrolysis of urea is unstable, especially at pH
8
when it dissociates and releases OH
ions. For a period of 1-2 days, the soil pH
around a urea granule rises as high as 9-10 and NH
3
volatilizes, according to the
reaction
NH
4
OH
NH
4
OH
NH
3
H
2
O (5.6)
When NH
4
breaks down and NH
3
gas escapes, the remaining H
ions neu-
tralize the OH
ions, so the overall process is pH-neutral. NH
4
ions that de-
compose in this way obviously are not oxidized to NO
3
, and hence do not con-
tribute to soil acidity. The loss of NH
3
is most serious when urea is applied to
the soil surface, and there is insufficient rain or irrigation to wash it into the soil.
High soil temperatures and wind accelerate NH
3
volatilization, and in extreme
cases up to 50% of the N in surface-applied urea can be lost.
Another process of gaseous N loss is
denitrification
. This occurs when part or
all of the soil becomes waterlogged, as discussed in section 5.6.1. Because vine-
yard soils should be well drained, and the fertilizer N inputs are relatively small,
denitrification losses should normally be small too.
5.4.1.3
Timing and Amount of N Fertilizer
The dynamics of vine N uptake are determined by shoot demand and root growth.
A generalized time trend for shoot and root growth, and N uptake from the soil,
in relation to phenological development of the vine, is shown in figure 5.6. Root
growth lags behind shoot growth from bud burst to early flowering. During this
period, N reserves in the roots and trunk are mobilized to the developing leaves,
and there is little uptake from the soil. The peak period of soil N uptake is from
full flowering to veraison, which coincides with the period of the most rapid root
growth. From veraison to harvest, the bunches are the main accumulators, with
most of the N coming from woody tissues and the shoots, and only a minor por-
tion from the soil. After harvest, N is accumulated in the permanent structures,
mainly by translocation from leaves and shoots; but up to one-third of the N may
be absorbed from the soil, more so in regions with long hot summers where the
roots continue to grow after harvest. This stored N provides the reserve for early
shoot growth during the following season and has an important influence on the
fruitfulness of the vines during that season. Given this pattern of N uptake, in
cool regions with significant spring rainfall, N fertilizer must not be applied
5.4.1.4
